Ionic centrifuge



1955 J. SLEPIAN 2,724,056

IONIC CENTRIFUGE Filed June 19, 1942 3 Sheets-Sheet l I I I I I 'l l l lM I WITNESSES: v INVENTOR 022 WZ'W JOJcj0/7 Sh p/Q27.

BY MM ATTOR NEY Nov. 15, 1955 J. SLEPIAN IONIC CENTRIFUGE 5 Sheets-Sheet2 Filed June 19, 1942 INVENTOR Joseph 5/6 2/0/1.

WITNESSES: owfiw BY WM W ATTORNEY Nov. 15, 1955 sLEPlAN 2,724,056

IONIC CENTRIFUGE Filed June 19, 1942 3 Sheets-Sheet 3 WITNESSES: IINVENTOR W 253M- J066jD/7 5/ 0/02 ATTORN EY United States Patent- IONICCENTR-IFUGE Joseph Slepian, Pittsburgh, Pa., assignor to WestinghouseElectric Corporation, East Pittsburgh, Pa., 2 corporation ofPennsylvania Application June 19, 1942, Serial No. 447,679

14 Claims. (Cl. 250-413) My invention relates to electric dischargeapparatus, and has particular relation to apparatus for separating theisotopes of substances. I

In accordance with the teachings of the prior art of which I am aware,isotopes have been separated by diffusion and by mechanical centrifugeand electromagnetic methods. For any of the methods, the rate ofenrichment is roughly proportionalto stances, such .as :uranium, in theisotope separation of V which .1 am particularly interested, law is ofthe order of 1% of W. For othersubstances, the relationship between AWand lcT is of the same order. Therefore, the enrich- "men-t factor perunit operation for diffusion separated methods is of the order of lessthan e and the rate of enrichment is small. Because the enrichment rateis small diffusion operations must be repeated an excessively "largenumber of times to effectively separate isotopes and, therefore, thediffusion apparatus is large and cumbersome.

In the mechanical centrifuge, with the highest speeds attainable, W .isapproximately equal to kT, so that for uranium, [the enrichment perstage isxof the order of e and again a very large number of :stages isnecessary.

Electromagnetically, isotopes are separated with a mass spectrograph orwith a magnetron. In either case, W can be as high as 10,000 151. Forsubstances, such as uranium, AW is, therefore, of the order of 100 kT,the enrichment factor is of the order of c 9, and the rate of enrichmentis large.

In separating isotopes with .a mass spectrograph, ions of the isotopesare projected into electric and magnetic fields. The masses of the ionsof the different isotopes are differout and, under the properconditions, thetrajectories of the ions of differentisotopes, as theymove :under the influence 10f forces exerted by the fields, are slightlydifferent. Separate electrodes are positioned in such manner as toreceive the ions moving in the different trajectories.

.It is essential ,for the operation of the 'mass spectrograph that ionsof different isotopes move in distinguish ably separate trajectories.This necessity imposes a limitation on the apparatus which isundesirable fromithe standpoint of isotope separation. To maintain thetrajectories separate, the ion current must be so small that the effectof space charge and collision between ions (on the ion current isnegligible. Experiments with the mass spectrograph have revealed thatthe ion current should be of theorder of at most a few :milliamperes forproper resolution of the trajectories in .a vessel of ordinarydimensions. For such currents'the deposition of thesepa rated materialis at a :minute rate. For somewhat larger 2,724,056 Patented Nov. 151955 current, there is a substantial loss in enrichment which faroutweighs the measured current in its effect on the yield.

In amagnetron, a source of the isotope ions is disposed at the center ofa conducting cylinder. A radial electric field is impressed between thesource and the cylinder, and a magnetic field perpendicular to theelectric field isapplied. Under the proper conditions, the electric andmagnetic fields may be so adjusted that the trajectories of the ions ofa lighter isotope return toward the source, while the trajectories ofthe ions of a heavier isotope ir'npinge on the cylinder. In this caseagain, the separation of the trajectories imposes the condition that theion current be so small that space-charge effects and collision betweenions are negligible. Experiments with the magnetron have revealed thathere again separation is accomplished only with a small ion current. Ation currents less than approximately one-half milliampere, theseparation is satisfactory.

if the current is increased to a magnitude somewhat greater thanone-half milliampere, for example, to a few milliamperes, a large lossin enrichment which far outweighsthe effect on the yield of increasingthe current occurs.

In the construction and the operation .of both the mass spectrograph andthe magnetron, high precision is required. The difference in the massesof the different isotopes of a substance is small, and the difference inthe effects of electric and magnetic fields on the ions of differentisotopes is correspondingly small. Small variations in the fields underwhich the ions move or in the spacing of the collecting electrodes may,therefore, entirely vit'iate the results.

It is accordingly an object of my invention to provide apparatus forseparating isotopes to obtain substantial quantities of the separatedsubstances.

Another object of my invention is to provide a method of separatingisotopes that shall have a high rate of en richment and that shall yieldsubstantial quantities of the separated substances.

A more specific object of my invention is to provide electromagneticapparatus for separating isotopes that shall operate with substantialion currents.

Another specific object of my invention is to provide electromagneticapparatus for separating isotopes, the construction and operation ofwhich shall not require high precision.

An ancillary object of my invention is .to provide a source of isotopeions.

A more specific ancillary object of my invention is to provide a sourceof ions of uranium isotopes.

More generally stated, it is an object of my invention to provideapparatus for separating isotopes that shall have the high rate ofenrichment of the electromagnetic method of separation and that shalloperate with substantial .masses of isotope ions.

Broadly stated, it is an object of my invention to provide apparatus forseparating ions of different masses.

In accordance with my invention, I provide an ionic centrifuge, that is,an isotope separator in which the separation is dependent on thedifference in the effects of centrifugal .forces of electric andmagnetic origin on the ions of different isotopes. My isotope separatoris of the electromagnetic type and has a relatively high enrichmentfactor. It comprises .a source of isotope ions disposed along the axisof a conducting cylinder and external radial electric and axial magneticfields cooperating to separate the ions. However, the operation of myseparator does not depend on the detailed resolution of trajectories ofdifferent isotope ions and neither space charge nor collision betweenions need be suppressed. On the contrary, in accordance with my'invencurrent is, therefore, substantial.

ions are substantially circles.

rent, it reaches a certain magnitude.

" tion, the space charge modifies the electric field impressed on theions'in such manner as to produce the desired centrifugal effects.

In the practice of my invention, the ion current is so i large thatthe'resultant of the space-charge field and the externally impressedelectric field exerts a force on the ions wihin any region which isgreater than the electric equivalent of the force exerted by themagnetic field for one of the isotopes and less than the electricequivalent for another and this over a considerable portion of spaceinstead of over a very limited region as in the magnetron. Under theaction of the resultant electric field and the magnetic field, the ionsof the last-mentioned isotopes move in, closely wound spirals, that is,spirals of small pitch, about the source of ions as a center. The ionsof the lighter of the two isotopes preponderantly move in spirals whichfirst recede from the source and then return toward the source; the ionsof the heavier of the isotopes 'preponderantly move in spirals whichrecede from the source until they impinge on the cylinder.

Since the space-charge effect contributes towards the operation of myseparator, it need not be suppressed and the ionic current may be large.The quantity of material enriched in an isotope which is deposited bythe Since the spirals in which the ions move are of small pitch, thepaths of the The angles between the paths of colliding ions are at thepoints of collision,

therefore, preponderantly small, and the collisions thus 7 do notmaterially increase the random energy, kT, of the ions. Their onlyeffect is to impart a common circumferential velocity to the collidingions. The centrifugal force exerted on an ion is directly proportionalto the product of its mass and the square of its circumferentialvelocity. equalize the circumferential velocity, the movement of Sincethe effect of the collisions is to the ions under the action of thecentrifugal force produced by coaction of the electric and magneticfields is to a large extent dependent only on the mass of the ions,

and ions which have collided and have not substantially the samecircumferential velocity lend themselves to separation in accordancewith their different masses. Rather than obstruct the separation processas they do in prior art apparatus, collisions in apparatus, according tomy invention, actually expedite the separation.

My invention arises from the realization that in an electromagneticsystem the enrichment does not decrease indefinitely as theionic currentis increased. I have found that as the ion current is increased theenrichment decreases at a high rate until, for a certain value of car-As the current is increased further, the enrichment does not decreasesubstantially below the magnitude. reaches the substantially constantvalue, the loss in enrichrnent far outweighs the effect of the increasedcurrent, and the net effect is a large loss in the rateof separation of'the material. After the constant enrichment value is reached, anyincrease in the ion current constitutes a net gain in the rate ofseparation, and for large enough ion currents, it may counterbalance theloss in enrichment.

To obtain a measure of the accomplishment of a separator, let M be themass of the material which the separator is capable of treating in asingle operation and E its isotopic enrichment, that is, the ratio ofthe masses of the isotopes in the treated substances after treatment tothe ratio of the masses of the isotopes before treatment. Compare theseparator with a unit separator having the same enrichment E, butcapable of treating the unit mass in a single operation. M unitseparators are required to produce a mass of enriched material, such asV is produced bythe separator under observation. The

total enrichment produced by M unit separators operatmg successively onthe same material is E. Therefore, the accomplishment of the separatorunder observation Until the enrichment.

4. V can be measured by the quantity E or the log of the quantity, thatis, M log E. e

In the mass spectrographs and magnetrons provided in accordance with theteachings of the prior art, M is maintained at a small value so that Eis large, but M log E is relatively small. As M is increased, log Edecreases at a higher rate, and the result is that M log E decreases.The electromagnetic separator, in accordance with my invention in itsbroader aspects, is based on the concept that if M is increased beyond acertain value, log Eremains substantially unchanged, and a furtherincrease'in M results in an increase in M log E.

The novel features that I consider characteristic of my invention areset forth with particularity in the appended claims- The inventionitself, however, both as to its organization and its method ofoperation, together with additional obiects and advantages thereof, Willbest be understood from the following description of a specificembodiment when read in connection with the accompanying drawings, inwhich:

Figure l is a diagrammatic view showing an embodiment of my invention;

Fig. 2 is a section taken along the line 11-1); of Fig. '1;

Fig. 3 is a graph illustrating the operation of the embodiment of myinvention shown in Fig. 1;

Fig. 4 is a diagrammatic view showing a modification of my invention;and

Fig. 5 is a graph illustrating the operation of the apparatus shown inFig. 4.

The apparatus shown in Figs. 1 and 2 comprises a substantiallyvacuum-tight container 7 having a circularly tight to the top plate 10in a position such that its axis is coextensive with the axis of theglass cylinder 8. The can has oppositely disposed openings 12 and 13,respectively,

in its side wall. A short tube 14 is welded vacuum-tight to the opening13. The tube 14 is connectedto a vacuum pump system (not shown) througha flexible hose 15' which engages the end of the tube.

Isotope ions are derived from an are 17 produced between anode andcathode electrodes 19 and 21, respectively, disposed at the center ofthe container 7. The cathode 2 1 is composed of the material, theisotopes of which are to be se arated. The anode 19 may be composed ofanother suitable material. Since I am primarily interes'ed in separatingthe isotopes of uranium, I provide a cathode of uranium metal or of asuitable uranium alloy or compound. I have found that a satisfactory arcis produced between a cathode 21 of uranium metal and an anode 19 ofcarbon.

When the apparatus is in operation, the are 17 is maintained between theanode and the cathode by repeatedly connecting and disconnecting the twoelectrodes. To accomplish this object, the cathode 21 is moved in andout of engagement with the anode 19 by the cooperation of a motoroperated cam 22 and an arm 23. At one end, the arm is urged intoengagement with the cam by a spring 24 which extends between the arm andthe rim of the top plate 10. At the other end, the arm 23 is pivoted toa rod 25 which carries the cathode 21. The rod 25 passes through anopening in a plug 26 which closes the lower end of the can 11. Theopening in the plug 26 is just large enough to permit the rod 25 toslide freely, but to prevent substantial lateral movement. I

At the point where the arm 23 enters the evacuated space, a vacuum-tightjoint is provided between the arm and the boundary of the space. The armpasses into the space through a rubber'plug 27 which is clamped tightlyto its side. The plug is mounted centrally on a circular bracket 28which is sealed to one end of a Sylphon 29. The other end of the Sylphonis sealed'in the remaining opening 12 of the can 11. The arm 23 and therod 25 may be properly positionedv by rotating a screw 30' which nmgosepasses through the rim of the bracket 28 and engages the can 11. n p l nThe stability of the ar'c,that is, the length of time for which it will.burn after it is struck, can be greatlyfincreased by continuallysupplying a small stream of arc stabilizing gas, preferably oxygen orair, to the neighborhood of the arc. Since the arc stabilizing gasimpairs in some degree the subsequent isotope separating effect, Iadjust its magnitude to as small a value as to give a reasonablestability to the arc. The gas is supplied through a perforation 33 inthe anode 19. The anode is mounted in 'ametal tube 34 which is sealedv'acum tight through the base 9. At the point where it passes throughthe base, the tube .34 is constricted and bent at right angles. The openend of tube 34 is engaged by a flexible hose 35 through which the arcstabilizing gas is transmitted. Near the mouth of tube 34, a wire 36 isclamped tightly between "the walls of the hose, providing a. constrictedopening through which the gas may leak slowly to the space between theelectrodes 19 and 21.

The ions in the are 17 are subjected to an accelerating radial electricfield which is impressed between a pair of coextensive conductingcylinders 39 and the cathode 21. Under the action of the field, the ionsare projected radially through a circumferential .slot 41 between thecylinders 39. On passing through the slot 41, the ions are subjected toa second accelerating radial electric field which is impressed between asecond set of coextensive conducting cylinders 43 and the first set.Under the action of the accelerating fields, the ions move through asecond circumferential slot 45 between the cylinders 43 into aring-shaped region 46 bounded by the latter cylinders, an externalconducting cylinder 47 having a height equal to the distance between theouter ends of the latter coextensive cylinders 43, and a pair ofring-shaped conducting side plates 49 extending between the edges of thecoextensive cylinders and the edges of the outer cylinder.

The ion streams produced bythe electric field are also subjected to amagnetic field, the lines of force of which are parallel tothe axis ofthe cylinders 7, '39, 43 and 47. The latter field is produced by anelecromagnet :51 having horizontal cylindrical poles 53 extending aboveand below the base 9 and top 10 of .theevacua'ted container 7. The poles53 are provided with suitable exciting windings 55 which are energizedfrom the source 56 or any other suitable direct-current source, and amagnetizable yoke 57, extends between them. The top 10 and base 9 of thecontainer 7 and the end plates 49 should be composed of a material whichpermits the passage of the magnetic lines of force from the poles 53through the ring-shaped space 46 bounded by the coextensive cylinders43, the external cylinder 47, and the side plates 49.

p The ionic current produced within the ring space 46 is of substantialmagnitude, and the space-charge effect produced by it so modifies theexternal electric field impressed on the ions that the resultantelectric 'fieldgi's at each point within the ring space greater than theelectric equivalent of the magnetic field for a heavier isotope and lessthan the electric equivalent for a lighter isotope. The ions of thelatter isotopes move in closely wound spirals, 'the ions of the lighterisotope first receding from thesource 17 and then returning to it, theside plates 49, or the coextensive cylinders 39 or 43, and the ions ofthe heavier isotope receding from the source until they impinge on thebounding cylinder 47. The apparatus may be operated for a substantialtime interval. Then the conductors bounding the ring space 46 may be removed from the container 7, and the substance enriched in one or theother of the isotopes may be removed from the Walls of the outercylinder 47, the coextensive cylinders 39m 43, or the side plates 49 towhich it has adhered. r v The-operation of the separator shown in Figs.1 and 2 is illustrated graphically in Fig. 3. -Inthis view, thecletci'omagnetic fields impressed 'on an ion are plotted as a functionof the radial position of the ion. The ion has a certain initial kineticenergy which is of a random character. The total additional kineticenergy imparted to .a singly ionized ion by the electric field at anypoint is Vs, where V is the potential of the electric field at thepoint, and e is the charge on an electron (that is, the charge on theion). The magnetic field causes some of the kinetic energy of the ionwhich is partly initial, and partly imparted by the electric field, tobe converted to energyof rotation about the central axis, thuscounteracting the effect of the electric field in producing energy ofpure radial motion and reducing the energy of purely radial motion. Themagnitude of the counteracting effect of the magnetic field is ii 22 Hrwhere m is the mass of the ion, H the magnetic field, and r the radialdistance of the point under consideration. The effect of the magneticfield may be regarded as produced by an equivalent electric field, thepotential of which is -HW volts 8 m which acts to drive the ion inwardlytoward the center.

' In Fig. 3, the field potential V, whether it be of elec trict ormagnetic origin, is plotted vertically, and the radial distance r isplotted horizontally. The light curve 59 represents the externallyimpressed electric field, such as would exist with the electrodearrangement of Fig. l, and in the absence of space charge, as a functionof the radial distance. The field is equal numerically to the externalpotential which is impressed and rises at a moderate rate from zero atthe source 17 until it reaches a substantial magnitude at the radialdistance ri correspending to the position of the external coextensivecylinders 43. Between the radial distance r1 and the position Tocorresponding to the external cylinder, the external field does not risesubstantially. In the absence of space charge, the light curve 59 wouldrepresent the actual potential impressed on the ions. The lower mediumweight curve 61 represents the electric potential equivalent of theaction of the magnetic field on the ions of the isotope which, tosimplify the explanation, I shall assume to be the heaviest of thetreated elements. The curve 61 is a parabola. The upper medium weightcurve 63 represents the electric potential equivalent of the magneticfield for a lighter isotope.

I shall first consider the situation which arises in prior artelectromagnetic separators in which space-charge and collision effectsare substantially absent. For such a separator the curve 59 representsthe actual potential of the electric field in which the ions move. Ifthe ions start with negligible initial velocity, the curve 59 alsorepresents the total kinetic energy which the ions have at any radius r.For values of r for which the electric potential exceeds the electricpotential equivalent of .the magnetic field, the ions have a finiteenergy of radial motion, and can move away from the center. At theradial distance corresponding to the point of intersection of the curveof electric potential and the potential equivalent of the magneticfield, the radial velocity of the ions is Zero, if they have made nocollisions previously as I have assumed. The ions, therefore,predominantly do not move beyond the radial distance corresponding tothe point of intersection of the curves, but return toward the are 17.Thus the light ions are predominantly turned back at the radial distancecorresponding to the intersection 64 of the curve 59 with the curve 63,while the heavy ions go on, and are not turned back until they reach theradial distance corresponding to the intersection 66 of the curve 59with the curve 61. If the outer collecting electrode is placed at adistance corresponding to the radial distancesbeuranium, it is of theorder of one percent of the radius of the separator. Thus high precisionis required in the construction of an electromagnetic separator in whichthe ions move in a space-charge-free field, and in such a separator theelectric and magnetic fields must be adjusted precisely to their propervalues and maintained uniformly at the proper values. Moreover, thecurrent of positive ions used in this separator must be kept smallenough so that with the degree of neutralization by electrons present,space-charge effects will not appreciably alter the electric field,since any such alteration of the electric field will shift the positionsof the intersections 64 and 66 of curve 59 with curves 63 and 61. Thecurrent of positive ions must also be kept small, so as to avoidcollision effects between ions. Such collisions will profoundly changethe trajectories of the ions, and completely change the radii at whichthe trajectories turn from the outward motion back toward the inwardmotion. This condition arises because the colliding ions will in thespace-charge-free region have large radial velocities, one outwardly,and the other inwardly.

I have found experimentally that space-charge and collision effectsbecome disturbing for small current of positive ions. For example, in amagnetron with a diameter of 16 inches for the outer electrode and witha' uranium ion current of only a milliampere or two, the space chargecaused many of the heavy isotope ions to turn back at a radial distancesubstantially smaller than the theoretical radial distance correspondingto the intersection points of the field curves.

In the ionic centrifuge which embodies the broad aspects of myinvention, space charge is permitted to develop to such a degree thatthe resultant electric po tential takes the form indicated by curve 65,that is, lies very close to the curves 61 and 63. An ion starting fromrest at the are 17 of an ionic centrifuge will have a total kineticenergy given by curve 65. at any radial distance. The circumferentialvelocity component of the energy will be given by curve 63 or 61,depending on whether it is alight or heavy isotope ion. The radialenergy of the ion which determines the radial velocity component isgiven by the difference between the total energy (curve 65) and thecircumferential energy (curves 61 and 63). Since the total energy curve65 lies near the circumferential curves 61 and 63, the difference issmall and the radial velocity is small compared .to the circumferentialvelocity. The ions, therefore, move over large circumferential arcswhile they are moving a short distance radially, and at any radius thepaths of the ions of both isotopes will be closely wound spirals. Thepoint of intersection 68 of. curve 65 with curve 63 corresponds to theradial position in the centrifuge at which V the ion of the lightisotope will stop spiraling outwardly, 'and will begin to spiralinwardly. The point of interof any; collision will spiral out until theyreach the radial position corresponding to the former inter ectionpoint.

In general, an ion will leave the center with a finite initial kineticenergy. In discussing prior art' electromagnetic separators, the initialenergy can be neglected, as it is small compared to. the energy which isimparted to the ions by the external electrical field. In the ioniccentrifuge, however, the initial kinetic energy may constituteasubstantial portion of thetotal energy and cannot be neglected.

If the initial energy is taken into consideration, Fig. 3 does notprecisely represent the actual conditions in'an ionic centrifuge. Thetotal kinetic energy of the ions is represented not by the curve 65, butby a plurality of curves displaced upwardly from curve by distancescorresponding to the initial kinetic energy of the ions. Theintersection points of the displaced curves 65 with curve 63 are shiftedto the right, and the turning points of the light isotope ions are atdifferent radial distances. The turning point of the spiral path of alight isotope ion with one initial energy occurs at a radius which islarger than that for a light isotope ion with a smaller initial energy.Since the distribution of initial energies of the ions is Maxwellian,the distribution of the turning points of the light ions is Maxwellian;that is, the radii at which the light isotope ions turn back aredistributed about amean radius exponentially, roughly in accordance withthe relationship r, where .e is the natural logarithm base and F is thedistance of any radius from the mean radius. Similarly, radii at whichthe heavy isotope ions turn' back are distributed about a mean radius inaccordance with the Maxwellian relationship. However, the latter meanradius is larger than the former. In accordance with my invention, theouter collector should be placed between the mean turning radii of thelight and heavy ions, so as to achieve a maximum separating effect ofthe isotopes being collected.

In a separator in accordance with my invention as contrasted to priorart electromagnetic separators, there is frequent collision betweenions, but the collisions do not appreciably impair the effectiveness ofthe apparatus in separating the ions. This condition arises because atany radius, the ions are moving circumferentially with nearly the samevelocity, while the radial velocities, although differing in magnitudeand direction, are small. 'fi'hen a collision occurs between two ions,their common circumferential velocityis preserved, and their radialvelocities are exchanged so that the paths of the ions taking part inthe collision are shifted to other paths. Each of the new pathscorresponds to the path of an ion similar to the colliding ion with adifferent initial energy. As a result of collisions, some ions will beshifted to paths corresponding to higherinitial energies, and some topaths corresponding to lower initial. energies. On thewhole, thetotality of paths of the ionswill not be changed substantially bycollisions, and, therefore, collisions will not materially impair theseparation..

In terms of its charge e, its mass m, the magnetic field H to which itis subjected and its radial position r, the energy of circumferentialmotion of the ions is given approximately by the expression In terms ofits mass and its angular velocity w about the axis of the container7,.the same energyis given bythe expression l/2mr w Equating the tworelationships, it follows that; V

w m n and is for each isotope constant at all radii. The rotation of theions isv thus the same as the rotation of a rigid body, and nodisturbances arise from the circumferential motion by rcason of the slipof the ions of one isotope at any radius relative to the ions at anadjacent radius. A small slip disturbance docs arise because the ions ofdifferent isotopes move at different angular velocities relative to eachother. v I

The motion of the ions in the space 46 is similar to that ofthe'molecules of a gas in a mechanical centrifuge.

The mass as a whole rotates like a rigid body, but the centrifugal'forces superimposed on the rigid rotation cause a preferentialdiffusion of the heavier molecules to the outer radii of the centrifuge,and a diffusion of the lighter molecules to the inner radii. 'It is forthis reason that Icall my device anionic centrifuge-meaning thereby thatthe ions undergo motions similar to the motions of the molecules of agas in a mechanical centrifuge. While my invention is of importancebecause of its application to the separation of isotopes, it is notlimited in this respect. Apparatus embodying the "broad concepts of myinvention may 'be applied to the separation of molecules of any type as,for example, molecules of different materials.

The operation of my ionic centrifuge depends on an ionic space-chargeeffect of proper magnitude and distribution. To explain how thespace-charge effect is developed, 1 shall first consider theoversimplified case in which the positive ions are the only chargedparticles .in the centrifuge. The positive charge carried by thepositive ions in transit from the source to the collecting electrodesforms a positive space charge whosefeifect upon the electric field andthe potential acting on the ions can be calculated by applying the usualelectrostatic theory. Qualitatively, the effect of the space-charge -isto reduce the potential in the space to a smaller value than theexternally impressed potential which is given by curve 59 in Fig. 3. Thespace-charge effect thus brings the space potential down from valuescorresponding to the curve 59 towards values corresponding to curves 63and 61 :(that is, towards curve 65) as the current ofpositive ionsavailable from the source is increased. At any point at which thepotential represented by curves 63 and 61 is equaled by the net spacepotential, ions are turned back, those with smaller initial kineticenergy being turned back if the net field is represented by a curvewhich is tangent to curve 63, and those with larger initial kineticenergies being turned back for field represented by a curve (such as 65)which penetrates the region 'of curves 63 and 61 more deeply.

The turning back of the ions lessens the space charge at larger radialdistances than those at which the ions are turned back, and increasesthe space charge at smaller radial distances. The decrease of the spacecharge beyond the turning back radial distances has the effect ofpreventing the space potential from sinking substantially below valuescorresponding to the region between the curves 63 and 61 at largerradial distances, and of causing the space potential to approach valuescorresponding to this region at smaller radial distances. Thus, as then'umber of available ions is increased at the source, the net potentialapproaches values corresponding to the region between curves 63 and 61throughout most of the space 46. (The net potential represented by thecurve which results when curve 59 is modified by space-charge effect--does in fact fall slightly below that represented by'curve :61 :forextreme radial distances. This condition arises because there are someheavy isotope ions in the space which have suflicient initial kineticenergy to be carried beyond radial distances corresponding to theintersection of curve 65 and curve 61 against the net field.)

The foregoing analysis is based on the assumption that only positiveions are present in the space 46. Under such circumstances, the spacechar ge effect would be large even for small ion currents, because thepositive ions move slowly. The space potential would 'then have valuesrepresented by points deep in the region of curves 63 and 61, andsubstantially all the ions set free at the source would be turned backthere, and only the few ions which have large initial velocities wouldpenetrate to larger radial distances, Thus, ,forthe .structure picturedin Fig. 1, with a sixteen-inch diameter outer collector 47 and with amagnetic field of 8000 gauss and an electric potential of 1200 volts,only a few m'icroampere's of positive ions would pass to the outerelectrode in the absence "to an extent.

In fact, there is space-charge neutralization within the region 46.Neutralizing effects arise from electrons which are abstracted bypositive ion bombardment from the metallic surfaces 49 which bound theionic centrifuge axially with respect to the magnetic field. The numberof electrons per ion emitted from the surfaces depends upon the natureof the surface, and the energy with which the ions strike the surfaceand is ordinarily small. 'The electron emission can be increased bycoating the surface with alkali or alkaline earth metals or theiroxides, or by insulating surfaces and impressing negative potentials onthem. However, only a small electron emission is needed to neutralizethe space charge of a relatively large ion current, because theelectrons remain in the radial plane where they are emitted. Afterleaving the surfaces 49, the electrons are able to move freely axiallyin the direction of the magnetic field, but are prevented fro rr' movingrapidly in a radial direction. The magnetic field converts any kineticenergy of radial motion which "the electrons may acquire as they leavethe: surfaces 49 into kinetic energy of circumferential motion.

The apparatus shown in Fig. 4 differs from that shown in Fig. 1 inseveral minor respects (certain parts, such as the magnet 51, are inFig. 4, omitted for the purpose of clarity).

The cam electrode drive in the latter modification is replaced by anautomatic electrode drive in the former. The are electrodes 19 and 21are supplied from the source 31 through the exciting coil of a relay 74.As long as there is an are between the electrodes 19 and '21, the relayis energized and pivots arm 23 downward against the action of a spring72 to maintain the electrodes separate. When the arc is interrupted,current flow through the coil of the relay ceases, and the arm 23 pivotsdownward so that the electrodes 19 and '21 are engaged and close thecircuit. The relay is now "reenergized, the electrodes are separated,and "an are between them is initiated.

The conducting side plates 49 bounding the ring space 46 in themodification shown in Fig. l are replaced by a plurality of separaterings 71 of diameters which 'progressively increase from the shells 43to the shell 47 in the Fig. 4 modification. The rings are supported frominsulating brackets 73 secured to the base 9 and the top plate, and aremaintained at potentials which become gradually more negative as theirdistance from the are 17 increases. The external fields impressed by thepotentials applied to the rings 73 thus approach the electricequivalents of the magnetic field for the isotopes more closely than inthe Fig. 1 embodiment, and the desired resultant field is produced moreeffectively by the space charge.

By varying "potentials impressed upon the rings 71, space-chargeneutralization occurring under each ring may be controlled, and thus thedepth to which the resultant space potential corresponds to the valuesin the region of curves 63 and 61 of Fig. 3 is controlled. If thepotential of a ring 71 is increased in magnitude, the electron emissionfrom it per ion striking it is increased, and the space-chargeneutralization in the space opposite it "is increased. The spacepotential in the space opposite it rises then above the values in theregion between the curves 63 and 61, and fewer ions are turned back inthe space. If, the potential of a ring 71 is reduced in magnitude, theelectron emission from it is reduced, the space-charge neutralizationopposite it is reduced, the space potential opposite it sinks to valueswhich correspond to "points deep in the region between curves 63 and 61,and more ions are turned back in the space opposite it. v

In practice, the potential of each of the rings 71 is decreased from alarge magnitude potential until further small reductions in potentialmagnitudes begin to affect the ion collection at the outer collector 47.At thispoint, the space potential begins to assume values correspondingto the region between curves 63 and 61. The potential magnitudes of eachof the rings is then lowered by small amounts until the collection atthe outer electrode is reduced by an amount corresponding to the turningback of all the light ions.

In Fig. 5, the operation of the Fig. 4 modification is illustratedgraphically. The medium curves 61 and 63 again represent the electricequivalents of the magnetic field forthe heaviest isotope and a lighterisotope. The light curve 75 represents the external electric fieldimpressed on the ions. The heavy curve 77 represents the resultantelectric field attained by combining the spacecharge field and theexternal electric field.

Although I have shown and described certain specific embodiments of myinvention, 1 am fully aware that many'modifications thereof arepossible. My invention, therefore, is not to be restricted exceptinsofar as is necessitated by the prior art and by the spirit of theappended claims.

I claim as my invention;

.1. For use in separating the isotopes of an element, the combinationcomprising a source of ions of the isotopes of said element, means forsubjecting said ions to an electric field radial about said source,means for subjecting said ions to a magnetic field substantiallyperpendicular to said electric field, a substantially cylindricalcollector for said ions with said source on its axis, the magnitudes ofsaid electric and magnetic fields being so related to the massof saidisotopes that the ions of one or" said isotopes'predominantly initiallymove away from said source and finally return in the direction of saidsource without reaching said collector and the ions of another of saidisotopes predominantly move away from said source and reach saidcollector, said electricfield being modified substantially by thespace-charge effect of said ions.

2. For use in separating the isotopes of an element, .the combinationcomprising asource of ions of the isotopes of said element, means forsubjecting said ions to an external radial electric field havingsubstantially circular symmetry about said source, means for subjectingsaid ions to a magnetic field at right angles to said electric field, acollector ,for said isotopes also having circular symmetry? about saidsource, the resultant of the field of the space charge of said ions andsaid external electric field being a field which, over a substantialrange of the distances from said source, is, at each point, not muchdifferent from the electric field equivalent of the magnetic field forone of said isotopes.

3. An electric discharge device comprsing a source of positiveions of amaterial having more than one isotope, and collecting means for saidpositive charges consisting of a hollowring of conducting material thebases of which are made up of insulated laminations of conductivematerial.

4. An electromagnetic isotope separator comprising a cylindricalenclosure having a central axis, a source of ions having more than oneisotope positioned substantially on said axis and means for adjustingthe mean value with respect to time of the electric potential withinsaid cylinder at a plurality of radii thereof.

5. An electromagnetic isotope separator, with ion source means andvoltage applying means acting to attract ions from said source to acollector electrode and the magnitudes of each said means beingsuflicient so that said separator may be operated beyond the minimum inM log E, Where M is the mass of the isotopic material treated, and E isthe enrichment factor;

6. An eiectromagnetic isotope separator, comprising a vacuum spacetraversed by a magnetic field and bounded by metallic surfaces certainof which are perpendicular to said magnetic field and have high electronemissive properties.

vacuum space in which ions of the isotopes move, and

means for adjusting at will a space charge neutralizing electronemission into the space. a

8. The method of operating an electromagnetic isotope separator, havingan ion source means and a voltage applying means subjecting said ions toan .electric field, which comprises making the flow of ions from saidsource and the magnitude of saidvoltage of the values which arenecessary and sufiicient so that said'separator operates beyond theminimum in its M log E characteristic where M is the mass of isotopicmaterial undergoing treatment in unit time and E is the enrichmentfactor for said material.

9. An electric discharge device comprising a source of positive gaseousions positioned in the axis of a cylindrical chamber the side walls ofwhich constitute a collecting electrode for said ions, at least one endwall for said chamber being made up of insulated annular conductors,means for inducing a magnetic field in said chamber parallel to saidaxis, and connections for impressing predetermined potentials on saidannular conductors relative to said source.

10. An electric discharge device comprising a source of positive ions ofa material having more than one isotope, collecting means for saidpositive charges consisting of a hollow ring of conductingmaterialhaving said source at its center, means for impressing anegative potential on said ring relative to said source, and partitionswhich are made up of insulated laminations of conductive material acrossthe respective end faces of said ring.

11. An electromagnetic isotope separator comprising a cylindricalenclosure having a central axis, a source'of ions having more than oneisotope positioned substantially on said axis, means for producing aradial electric field in the space between said source and saidenclosure, and means for adjusting the mean value with respect to timeof the electric potential within said enclosure at a plurality of radiithereof.

12. An electromagnetic isotope separator comprising acylindricalenclosure having a central axis, a source of ions having morethan one isotope positioned substanelectric field between said sourceand the walls of said enclosure, and means for adjusting the mean valuewith respect to time of the electric potential of said electric 'fieldat a plurality of radii thereof.

14. An electromagnetic isotope separator comprising a cylindricalenclosure having a central axis, a source of ions having more than oneisotope positioned substantially on said axis, means for'producing anaperiodic radial electric field between said source and the walls ofsaid enclosure, means for producing a magnetic field substantiallyparallel to said axis within said enclosure, and means for adjusting themean value with respect to time of the electrical potential of saidelectric field at a plurality of radii thereof.

References Cited in the file of this patent UNITED STATES PATENTS KingApr. 18, 18 82 Snook May 17, 1927 (Other references on following page)13 UNITED STATES PATENTS Slepian Oct. 11, 1927 Lawrence Feb. 20, 1934Muller Dec. 4, 1934 Hollrnann Mar. 28, 1939 5 Kuhn et a1 Oct. 22, 1940Bleakney Nov. 12, 1940 Jonas Jan. 21, 1941 14 2,252,508 Hoff Aug. 12,1941 2,258,149 Schutze Oct. 7, 1941 2,261,569 Schutze Nov. 4, 1941 OTHERREFERENCES Physical Review, vol. XI, No. 4, pages 316325. Oliphant eta1.: Proceedings Royal Society of London (1934), V146A. Pages 922929.

1. FOR USE IN SEPARATING THE ISOTOPES OF AN ELEMENT, THE COMBINATIONCOMPRISING A SOURCE OF IONS OF THE ISOTOPES OF SAID ELEMENT, MEANS FORSUBJECTING SAID IONS TO AN ELECTRIC FIELD RADIAL ABOUT SAID SOURCE,MEANS FOR SUBJECTING SAID IONS TO A MAGNETIC FIELD SUBSTANTIALLYPREPENDICULAR TO SAID ELECTRIC FIELD, A SUBSTANTIALLY CYLINDRICALCOLLECTOR FOR SAID IONS WITH SAID SOURCE ON ITS AXIS, THE MAGNITUDES OFSAID ELECTRIC AND MAGNETIC FIELDS BEING SO RELATED TO THE MASS OF SAIDISOTOPES THAT THE IONS OF ONE OF SAID ISOTOPES PREDOMINANTLY INITIALLYMOVE AWAY FROM SAID SOURCE AND FINALLY RETURN IN THE DIRECTION OF SAIDSOURCE WITHOUT REACHING SAID COLLECTOR AND THE IONS OF ANOTHER OF SAIDISOLOPES PREDOMINANTLY MOVE AWAY FROM SAID SOURCE AND REACH SAIDCOLLECTOR, SAID ELECTRIC FIELD BEING MODIFIED SUBSTANTIALLY BYTHESPACE-CHARGE EFFECT OF SAID IONS.